1. Field of the Invention
The present invention relates to an image formation system such as an electro-photographic copying machine, laser printer, and the like, specifically to the image formation system suitable for forming images of a high image quality.
2. Description of the Related Art
In recent years, in the image formation system such as a printer, copying machine, facsimile, and the like, has been proposed the so-called full-color tandem machine, with a view to forming color images at a high speed and in a high image quality. Here, a system can be cited as a typical one of this tandem machine, in which: four image formation units of yellow (Y), magenta (M), cyan (C), and black (B) are arranged in cascade; the toner images of the four colors, yellow, magenta, cyan, and black that are sequentially formed by these image formation units are primarily transferred onto an intermediate transfer belt as an intermediate transfer member, thereafter the toner images are secondarily transferred all at once onto a sheet from the intermediate transfer belt; and the toner images formed on this sheet are fixed, whereby a full-color or monochromatic image is formed.
In this type of the image formation system, the setting of various image formation conditions is carried out to form satisfactory images on a sheet. Conventionally, a method of setting the primary transfer condition being one of the image formation conditions has be proposed, which includes: forming test patches on the intermediate transfer belt, while varying the strength of the primary transfer field in each image formation unit; and setting the optimum strength of the primary transfer field on the basis of the detection results of the patched toner images by a density sensor located to face to the intermediate transfer belt (refer to Patent Reference 1).
[Patent Reference 1]
Japanese Published Unexamined Patent Application No. Hei 11-212315.
However, the density sensor only detects the image density of a partial region along the conveyance direction of the intermediate transfer belt. Accordingly, it has been difficult to detect the imperfections of the image quality that are created on the edges of the sheet. For example, it is possible to prevent a smear from emerging on the rear part of the images on the sheet, by setting the strength of the transfer field slightly lower than the normal setting value. However, if the smear is not present in the region to be detected by the density sensor, the strength of the transfer field will be set to the normal setting value, accordingly it is impossible to prevent imperfections of the image quality from occurring.